This invention relates to position determination systems. Specifically, the present invention relates to an apparatus and method for generating and transmitting signals that are used for accurately determining position and a method and apparatus for determining position.
The U.S. Department of Defense has developed a Global Positioning System (GPS) that consists of a constellation of orbiting satellites that transmit a GPS signal. The GPS signal includes a P-code, a C/A code, and a GPS navigation message that are used by receivers for determining position. More particularly, P-codes and the GPS navigation message are used by authorized users (e.g. the U.S. military) for determining position while C/A codes and the GPS navigation message are used by civilians for determining position. The GPS navigation signals commonly available to civilian users are referred to as the standard positioning service (SPS).
Use of the GPS for precision agriculture, surveying, construction, vehicle location, etc. has grown in recent years. However, many applications require a higher accuracy than is available using the SPS. More particularly, atmospheric conditions introduce error into the position determination process. Also, the U.S. Air Force intentionally introduces error into the transmitted ephemerides and clock.
Reference stations, typically referred to as differential GPS (DGPS) reference stations, transmit correction data that can be used for accurately determining position. These reference stations are typically dedicated facilities with permanent and extensive broadcast capabilities. Each reference station transmits either their calculated corrections to the GPS signals or their actual observations of the GPS signals (raw data), or both.
One method for obtaining accurate position that compensates for intentionally induced error and error due to atmospheric conditions is known as Differential GPS (DGPS). DGPS position determination systems use calculated corrections to the GPS data broadcast from a reference station to accurately determine position. The calculated corrections to the GPS data that is used for determining position using DGPS techniques is commonly referred to as code phase correction data.
More recently, Real Time Kinematic (RTK) systems have been developed that use carrier phase techniques for accurately determining position. More particularly, each RTK position determination system receives the observations of the GPS signal and removes errors associated with atmospherics and satellite timing/clock errors at the RTK system. To accommodate RTK systems, some reference stations now transmit observations of the GPS signals (raw data) for each satellite which is used by RTK systems for accurately determining position. The observations of the GPS signals that are used for determining position using RTK techniques are commonly referred to as carrier phase observable data.
Many reference stations broadcast correction data that conforms to the Radio Technical Commission for Maritime services (RTCM) standard. The RTCM standard includes format standards, communication bands, and messages for a differential correction GPS service. The US Coast Guard operates and maintains regional reference stations that calculate and broadcast correction data that complies with the RTCM standards. The RTCM correction data broadcast by some US Coast Guard reference stations includes carrier phase observable data for position determination using RTK methods while data broadcast by other facilities only includes only code phase correction data. However, irrespective of whether the particular US Coast Guard facility broadcasts carrier phase data or code phase correction data, the broadcast is typically in a standard RTCM format.
Though there are many reference stations dedicated to serving the maritime industry, many coastal areas do not have a nearby reference station. Also, few inland areas are covered by a reference station. Therefore, there is a need for reference stations that broadcast correction data, particularly in inland areas.
As use of the GPS for precision agriculture, surveying, construction, vehicle location, etc. has grown, the need for reference stations that broadcast correction data has grown. However, reference stations that transmit correction data are expensive to build. Land must be acquired, a building or buildings must be erected, and a radio tower must be constructed. Then, power must be provided to the site and the necessary electronics equipment must be installed. Also, the facility must be maintained and in many cases continually manned.
What is needed is a method and apparatus for providing widely available correction signals that can be used by position determination systems for determining position. This method and apparatus should be compatible with current GPS receivers. Also, a method and apparatus is needed that uses correction signals for accurately determining position. The present invention provides a solution to the above needs.
The present invention provides a method and apparatus for providing widely available correction signals that can be used by position determination systems for determining position. This method and apparatus is compatible with current GPS receivers Also, the present invention provides a method and apparatus that uses the correction signals for accurately determining position.
In one embodiment of the present invention, a radio station modification unit is described that includes a beacon signal generator and a diplexer that is electrically coupled to the beacon signal generator. The beacon signal generator produces a signal that includes correction data. The radio station modification unit is designed to be incorporated into an existing radio transmission facility such as, for example an AM radio station.
In one embodiment of the present invention, the radio station modification unit is installed in an existing AM radio station by placing the beacon signal generator in a location where position determining signals such as, for example, signals from the US Global Positioning System (GPS) can be received. The diplexer is then coupled to the AM radio station""s transmitter and is connected to the AM radio station""s antenna. The diplexer outputs the radio station""s AM signal and a second signal, hereinafter referred to as a correction data signal. The AM signal and the correction data signal are radiated by the AM radio station""s antenna.
The AM signal is radiated at the same frequency that the radio station was using prior to the installation of the radio station modification unit. Therefore, the apparatus and method of the present invention does not significantly alter the station""s AM signal. Thus, other than the inconvenience of installation, there is no interference with the operation of the AM radio station. However, a slight power degradation may result. In one embodiment, the correction data signal is radiated at a frequency in the range of 200 to 400 kilohertz. In the present embodiment, a frequency of 300 kilohertz (300 kHz) is used.
The correction data signal includes correction data that can be used by users having properly equipped position determination systems for accurately determining position. Because many existing DGPS equipped and RTK equipped position determination systems are designed to receive correction data signals in the 300 kHz frequency band, by transmitting the correction data signal in the 300 kHz frequency band, no modification to existing position determination systems is required.
By integrating the radio station modification unit of the present invention with an existing radio transmission facility such as an existing AM radio station, a correction data signal is easily and inexpensively provided. Moreover, because existing radio transmission facilities such as AM radio stations are used, there is no need to construct a dedicated building and radio tower as is required in the prior art.
AM radio stations are numerous and AM radio stations are located in many regions that require accurate position determination. Therefore, by combining the radio station modification unit of the present invention with these existing AM radio stations, correction data signals can be received in numerous regions that now do not have access to correction data.
In one embodiment of the present invention, users of correction data are charged based on usage. Radio signals are broadcast that include correction data that is encrypted from an existing radio station. Upon payment by a user for use of correction data, an activation instruction is included in the radio signal transmitted from the existing radio station. The activation instruction instructs a particular position determination system to decrypt the encrypted correction data and to use received correction data to accurately determine position. This allows for efficiently and easily charging users based on usage.
In one embodiment, position determination systems that include a radio receiver and that can decrypt received encrypted correction data are provided to users. In the present embodiment, the radio receiver continually monitors one or more frequency to receive correction data signals. The position determination system functions as a standard GPS receiver, determining position using satellites or other available sources of position determination signals until such time that an activation instruction is received. The activation instruction engages the position determination system to accurately determine position using correction data signals.
In one embodiment, as long as the user pays for the use of correction data, the position determination system accurately determines position by decrypting and using correction data signals. Once the user""s subscription is over, a deactivation instruction is included in the radio signal broadcast from the existing AM radio station. The deactivation instruction instructs the particular position determination system to discontinue decryption and usage of correction data signals.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments that are illustrated in various drawing figures.